Mine seals are used extensively in underground coal mines to segregate mined-out areas and to isolate fire zones or underground areas that are susceptible to spontaneous combustion. Over the years, 30,000 mine seals have been erected in U.S. underground coal mines. Mine seals, along with rock dusting and ventilation, are the fundamental means of preventing underground coal mines explosions. In this study, NIOSH used ground penetrating radar (GPR) technology to determine if each of three test mine seals was uniformly constructed. Three mine seals representing different construction methods and materials (cast-in-place foamed cement; solid-block, polyurethane foam, and gravel; and wire-mesh, rebar, and gunite) were erected at the NIOSH Lake Lynn Laboratory's underground mine as part of an ongoing research program. The seals were imaged using GPR with 400-, 500-, 900-, and 1,000-MHz antennas. We found that variations in uniformity existed in the material for the cast-in-place foamed cement seal. Variations in uniformity also existed in the seal made from solid block, polyurethane foam, and gravel. The postprocessed radar records correlated closely with recovered core samples (areas of gravel and polyurethane foam, and areas containing only polyurethane foam). Finally, the radar records of the seal made of wire mesh, rebar, and gunite showed the various components. The results of this work suggest that GPR could be used as a tool to evaluate mine seal characteristics and construction uniformity.